There is a new law restricting protests deemed illegal, and several opposition leaders are facing criminal charges.

Some demonstrators released colourful balloons decorated with ski masks like the ones worn by the Pussy Riot punk band, three of whose members have been jailed for singing a profanity-laced protest song against Mr Putin in a church.

Witnesses say opposition leaders appear to have achieved their goal of attracting at least 50,000 people - enough to maintain the momentum of their nine-month-old movement but almost certainly too few to increase alarm in the Kremlin.

"The summer has gone, three months since our last march. Not a single demand has been met ... on the contrary, repressions have only gathered pace, more people have been arrested," far-left leader Sergei Udaltsov told a rally after the march.

Recalling a stunt in which Mr Putin flew in a light aircraft alongside migrating cranes this month, he said: "The president has detached himself from reality. He flies with cranes and just spits on the people from above."

Organisers also released white balloons and doves into the cloudy sky before opposition leaders led the march down a leafy central Moscow boulevard behind a long banner declaring: "For early elections! Against repression!"

Some wore T-shirts demanding the release of 17 protesters facing trial over a rally on May 6 that ended in clashes with police.

Others said it was vital to keep attending protests to inspire others to join them and to defy the Kremlin's pressure.

"We will come here as if to work. It must be part of our everyday life," said Alexei Navalny, an opposition blogger and protest organiser.

It must be part of our everyday life. I want each of us to ask ourselves when we look in the mirror in the morning what we can do for freedom.

Protest organiser Alexei Navalny

"I want each of us to ask ourselves when we look in the mirror in the morning what we can do for freedom."

Riot police were out in force but stood by watching for most of the day.

Witnesses said they detained Mr Udaltsov and several others around 10.00pm (local time) - the time by which city authorities had said the eight-hour protest must end.

The protesters say Mr Putin's return to the Kremlin after four years as premier is a setback for democracy. He could now extend his rule of Russia to 24 years if he wins another term when his mandate expires in 2018.

This would mean ruling longer than Soviet leader Leonid Brezhnev, and opponents fear political and economic stagnation.

There was also more focus on social problems than at previous protests, and more communists marched this time with nationalists, liberals, gay rights campaigners and leftists.

Other marchers wanted to show solidarity, without being confident the opposition can achieve much after failing to select a leader or touch off protests outside the big cities.

The demonstrations began last December over allegations of fraud in a parliamentary election won by Mr Putin's party and turned into the biggest protests against him since he first became president in 2000, at times drawing up to 100,000 people.

Mr Putin, who is 60 next month, dismisses the protesters as a minority who do not have wide support across Russia.

Apart from some minor electoral reforms at the peak of the protests, the Kremlin has resisted calls for political change.